Abstract. A large number of activities have been carried out during the last decade in different regions of the world, including polar regions, aiming to assess the level of mercury (Hg) species in ambient air and in precipitation observing their variation over time and with changing meteorological conditions. Following the discovery of atmospheric Hg depletion events (AMDEs) in Polar Regions several studies have indeed been conducted in order to assess the chemical-physical mechanisms related to AMDEs occurred in polar atmospheres with special attention to the consequences of these phenomena in terms of contamination of polar environment due to the rapid conversion of atmospheric gaseous Hg (Hg0) into reactive and water-soluble forms that may potentially become bioavailable. The understanding of the way in which mercury released to the atmosphere is eventually incorporated into biota is of crucial importance not only for the polar regions but also for the marine environment in general. The world's oceans and seas are in fact both sources and sinks of Hg and although it appears that the atmosphere is the major transport/distribution medium for Hg, because most Hg emissions are to the atmosphere, oceans and seas also play an important role. Currently, however, a coordinated observational network for Hg does not exist. There are a number of state and national programs that are collecting atmospheric Hg data but the parameters monitored, the locations of the monitoring sites and the methods employed may prohibit their utility in assessing Hg long-trend variations. The large increase in mercury emissions in fast developing countries (i.e., China, India) over the last decade due primarily to a sharp increase in energy production from the combustion of coal are not currently reflected in the long-term measurements of total gaseous mercury in ambient air and in precipitation data at several continuous monitoring sites in North Europe and North America. The discrepancy between observed gaseous mercury concentrations (steady or decreasing) and global mercury emission inventories (increasing) is not yet clear however, could be at least in part accounted by the increasing in the potential oxidation of the atmosphere recently documented. Therefore, measurements of other key atmospheric constituents at the global monitoring sites are necessary for us to develop a better understanding of the global redistribution of Hg and to further refine model parameterizations of the key processes. The sharing of data from this network, allowing, in fact, access to comparable and long-term data from a wide array of locations for understanding temporal and spatial patterns of Hg transport, deposition and re-emission process producing thus data that will support the validation of regional and global atmospheric Hg models. This paper presents a detailed overview of atmospheric mercury measurements conducted in the Northern and Southern Hemispheres at several terrestrial sites (industrial, rural and remote) during the last decade as well as measurements performed over the world's ocean and seas and in Polar Regions with reference to the monitoring techniques and location of monitoring sites in most of the continents.
Spatial and temporal trends in runoff at long-term streamgages within and near the Chesapeake Bay Watershed
